The effort to designate the Mountains to Sound Greenway — a 100-mile greenbelt along Interstate 90 — as a National Heritage Area received a key endorsement from a King County Council committee Tuesday.

The designation from Congress is meant to highlight a unique feature or local history. The greenway could be the only National Heritage Area in Washington; no other region is designated as such. (Nationwide, Congress has designated 49 heritage areas from coast to coast.)

Council Transportation, Economy and Environment Committee members sent the full council a recommendation to pass a measure urging Congress to designate the greenway as a National Heritage Area. The council is expected to act on the measure March 19.

The greenway stretches from the Seattle waterfront and across the Cascades to Kittitas County. The landscape includes more than 750,000 acres for education, recreation and environmental conservation.

“I spent much of my youth hiking in the Mountains to Sound Greenway with my brother and mom,” Issaquah-area Councilman Reagan Dunn, prime sponsor of the motion, said in a statement. “It is truly one of the jewels of the Northwest. Congress should ensure its protection by designating the Mountains to Sound Greenway as a National Heritage Area.”

National Park Service officials describe heritage areas as “lived-in landscapes” — not as national parks, though the agency does handle oversight for the heritage areas. The designation does not add lands, land-use restrictions or more regulatory authority inside the National Heritage Area.

In 1991, after a march from Snoqualmie Pass to Seattle a year earlier to raise awareness, citizen, conservation, corporate and government interests formed the Mountains to Sound Greenway Trust to act as a shepherd for the greenbelt.

In 1998, Federal Highway Administration officials designated the 100-mile greenway as the inaugural National Scenic Byway.

“I have worked throughout my career to help secure conservation lands along the Mountains to Sound Greenway,” said Councilman Larry Phillips, the committee chairman and a member of the Mountains to Sound Greenway Advisory Council. “These lands are an important part of preserving what is beautiful, green, and unique about our region. The greenway deserves the distinction of being designated a National Heritage Area.”